Semiconductor device and method of forming the same

ABSTRACT

Provided is a memory device including a first gate, a second gate and an inter-gate dielectric layer. The first gate is buried in a substrate. The second gate includes metal and is disposed on the substrate. The inter-gate dielectric layer is disposed between the first and second gates. A method of forming a memory device is further provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of China patent applicationserial no. 201510521058.5, filed on Aug. 24, 2015. The entirety of theabove-mentioned patent application is hereby incorporated by referenceherein and made a part of the specification.

BACKGROUND OF THE INVENTION

Field of Invention

The present invention is related to a semiconductor device and a methodof forming the same, and more generally to a memory device and a methodof forming the same.

Description of Related Art

With the development of a multi-functional chip, integrating elementswith different functions, e.g., a memory and a metal-oxide-semiconductor(MOS) transistor, into the same chip has become the mainstream in themarket. However, the process for fabricating a memory is commonlyseparated from the process for fabricating a MOS transistor. Hence,multiple photo-masks and complicated process steps are required, so asto increase the process cost and weaken the competitiveness. Therefore,how to effectively integrate a memory and a MOS transistor has beendrawn high attention in the industry.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a memory device and a methodof forming the same, in which a memory device can be fabricated at thesame time during the process of forming a metal gate, so as toeffectively integrate different elements with different functions into asingle chip.

The present invention provides a memory device including a first gate, asecond gate and an inter-gate dielectric layer. The first gate is buriedin a substrate. The second gate includes metal and is disposed on thesubstrate. The inter-gate dielectric layer is disposed between the firstand second gates.

According to an embodiment of the present invention, a dimension of thesecond gate is greater than a dimension of the first gate, and theinter-gate dielectric layer is further disposed between the second gateand the substrate.

According to an embodiment of the present invention, the inter-gatedielectric layer includes an oxide-nitride-oxide (ONO) dielectric layer,a high-dielectric-constant (high-k) layer having a dielectric constantof greater than about 10 or a combination thereof.

According to an embodiment of the present invention, the high-k layerincludes metal oxide.

According to an embodiment of the present invention, the memory devicefurther includes a tunnel insulating layer disposed between the firstgate and the substrate.

According to an embodiment of the present invention, the memory devicefurther includes at least two doped regions disposed in the substratebeside the first gate.

According to an embodiment of the present invention, a depth of thefirst gate is greater than a depth of the doped regions.

The present invention further provides a memory device including a firstgate, a second gate and an inter-gate dielectric layer. The first gateis buried in the substrate. The second gate includes metal and isdisposed on the substrate. The inter-gate dielectric layer is disposedbetween the first gate and the second gate, wherein the inter-gatedielectric layer includes a high-k layer having a dielectric constant ofgreater than about 10.

According to an embodiment of the present invention, a dimension of thesecond gate is greater than a dimension of the first gate, and theinter-gate dielectric layer is further disposed between the second gateand the substrate.

According to an embodiment of the present invention, the memory devicefurther includes an interfacial layer disposed between the high-k layerand the first gate.

According to an embodiment of the present invention, the memory devicefurther includes a tunnel insulating layer disposed between the firstgate and the substrate.

According to an embodiment of the present invention, the memory devicefurther includes at least two doped regions disposed in the substratebeside the first gate.

According to an embodiment of the present invention, a depth of thefirst gate is greater than a depth of the doped regions.

The present invention also provides a method of forming a memory device.A substrate having a first area and a second area is provided. Thesubstrate has at least one opening formed in the first area. Aninsulating layer is formed on a surface of the opening. A firstconductive layer is formed in the opening. A first dielectric layer anda second conductive layer are formed on the first conductive layer.

According to an embodiment of the present invention, the method furtherincludes forming a second dielectric layer around the second conductivelayer, removing the second conductive layer to form a trench in thesecond dielectric layer, and filling a third conductive layer in thetrench.

According to an embodiment of the present invention, the secondconductive layer includes polysilicon, amorphous silicon or acombination thereof, and the third conductive layer includes metal.

According to an embodiment of the present invention, the thirdconductive layer in the first area and a metal gate in the second areaare formed simultaneously.

According to an embodiment of the present invention, the firstdielectric layer includes an ONO dielectric layer, a high-k layer havinga dielectric constant greater than about 10 or a combination thereof.

According to an embodiment of the present invention, a surface of thefirst conductive layer is lower than a surface of the substrate.

According to an embodiment of the present invention, the firstdielectric layer in the first area and a high-k layer below a metal gatein the second area are formed simultaneously.

In view of the above, in the memory device of the invention, a floatinggate is buried in the substrate, and a control gate is fabricatedsimultaneously and formed at the same levels with a metal gate in aperiphery area are. In the present invention, a memory device and a MOStransistor device can be easily integrated together with the existinghigh-k and metal gate process, so the process cost is significantlyreduced and the competiveness is greatly improved.

In order to make the aforementioned and other objects, features andadvantages of the present invention comprehensible, a preferredembodiment accompanied with figures is described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1A to FIG. 1H are schematic cross-sectional views of a method offorming a semiconductor device according to an embodiment of the presentinvention.

FIG. 2 is a schematic cross-sectional view of a semiconductor deviceaccording to another embodiment of the present invention.

FIG. 3 is a schematic cross-sectional view of a semiconductor deviceaccording to yet another embodiment of the present invention.

FIG. 4 is a schematic cross-sectional view of a semiconductor deviceaccording to still another embodiment of the present invention.

FIG. 5A to FIG. 5E are schematic cross-sectional views of a method offorming a semiconductor device according to another embodiment of thepresent invention.

FIG. 6 is a schematic cross-sectional view of a semiconductor deviceaccording to yet another embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeelements.

FIG. 1A to FIG. 1H are schematic cross-sectional views of a method offorming a semiconductor device according to an embodiment of the presentinvention.

Referring to FIG. 1A, a substrate 100 is provided. The substrate 100 canbe a semiconductor substrate, such as a silicon-containing substrate.The substrate 100 has a first area 10 and a second area 20. In anembodiment, the first area 10 can be a cell area, second area 20 can bea periphery area. In another embodiment, the first area 10 can be amemory device area, and the second area 20 can be a MOS device area or alow-voltage device area, but the present invention is not limitedthereto. In an embodiment, the substrate 100 has a pad oxide layer 101and a mask layer 102 formed thereon. The pad oxide layer 101 includessilicon oxide, and the forming method thereof includes preforming athermal oxidation process. The mask layer 102 includes silicon nitride,and the forming method thereof includes performing a suitable depositionprocess such as a chemical vapour deposition (CVD) and a subsequentpatterning step such as a photolithography and etching process.Thereafter, a portion of the substrate 100 is removed by using the masklayer 102 as a mask, so as to faun at least one opening 104 in thesubstrate 100 in the first area 10. The method of removing the portionof the substrate 100 includes performing an etching process.

Referring to FIG. 1B, an insulating layer 106 is formed on the surfaceof the opening 104. The insulating layer 106 includes silicon oxide, andthe forming method thereof includes performing a thermal oxidationprocess. Afterwards, a conductive layer 108 is filled in the opening104. In an embodiment, the surface of the conductive layer 108 issubstantially coplanar with the surface of the mask layer 102. Theconductive layer 108 includes polysilicon, amorphous silicon or acombination thereof. The method of forming the conductive layer 108includes performing a suitable deposition process (e.g., CVD) to form aconductive material layer (not shown) on the substrate 100 filling inthe opening 104. Thereafter, a chemical mechanical polishing (CMP)process is performed by using the mask layer 102 as a polish stop layer,so as to remove the conductive material layer outside of the opening104.

Referring to FIG. 1C, a portion of the conductive layer 108 is removed,so the surface of the remaining conductive layer 108 a is no higher thanthe surface of the substrate 100. In an embodiment, the surface of theconductive layer 108 a is lower than the surface of the substrate 100,as shown in FIG. 1C. However, the present invention is not limitedthereto. In another embodiment, the surface of the conductive layer 108a is substantially coplanar with the surface of the substrate 100. Themethod of removing the portion of the conductive layer 108 includesperforming an etching back process. The mask layer 102 is then removed.

Referring to FIG. 1D, an oxide-nitride-oxide (ONO) dielectric layer 110is formed on the substrate 100 in the first area 10. In an embodiment,the distance from the surface of the conductive layer 108 a to thesurface of the substrate 100 is substantially equal to the thickness ofthe ONO dielectric layer 110, so the surface of the ONO dielectric layer110 above the conductive layer 108 a is substantially coplanar with thesurface of the substrate 100. The method of forming the ONO dielectriclayer 110 includes performing multiple deposition processes (e.g., CVD),so as to form an ONO dielectric material layer (not shown) on thesubstrate 100 in the first area 10 and in the second area 20.Thereafter, the ONO dielectric material layer in the second area 20 isremoved. In an embodiment, a photoresist layer (not shown) is formed onthe ONO dielectric material layer to cover the first area 10 and exposethe second area 20. Thereafter, the ONO dielectric material layerexposed by the photoresist layer is removed.

Afterwards, at least two doped regions 113 are formed in the substrate100 beside the conductive layer 108 a. The method of forming the dopedregions 113 includes performing an ion implantation process. In anembodiment, the depth of the doped regions 113 is less than the depth ofthe conductive layer 108 a. Besides, the doped regions 113 is in contactwith the sidewall of the opening 102. In the said embodiment, the ONOdielectric layer 110 is formed prior to the formation of the dopedregions 113, but the present invention is not limited thereto. Inanother embodiment, the ONO dielectric layer 110 can be formed after theformation of the doped regions 113. Thereafter, an etching process isperformed to remove the pad oxide layer 101 on the substrate 100 in thesecond area 20.

Referring to FIG. 1E, an interfacial layer 112 is formed on thesubstrate 100 in the second area 20. The interfacial layer 112 includessilicon oxide. In an embodiment, when the interfacial layer 112 isformed by a thermal oxidation process, the interfacial layer 112 isformed on the substrate 100 merely in the second area 20. In anotherembodiment, when the interfacial layer 112 is formed by a depositionprocess such as CVD, the interfacial layer 112 is formed on the ONOdielectric layer 110 in the first area 10 and on the substrate 100 inthe second area 20.

Thereafter, a high-dielectric-constant (high-k) layer 114 is formed onthe substrate 100 in the first and second areas 10 and 20. In anembodiment, the high-k layer 114 covers the ONO dielectric layer 110 inthe first area 10 and the interfacial layer 112 in the second area 20.The method of forming the high-k layer 114 includes performing asuitable deposition process such as CVD. In an embodiment, the high-klayer 114 can be a high-k layer with a dielectric constant greater thanabout 4, greater than about 7 or greater than about 10. For example, thehigh-k layer 114 includes metal oxide, such as rare earth metal oxide.The high-k material can be selected from the group consisting of hafniumoxide (HfO₂), hafnium silicon oxide (HfSiO₄), hafnium silicon oxynitride(HfSiON), aluminum oxide (Al₂O₃), lanthanum oxide (La₂O₃), tantalumoxide (Ta₂O₅), yttrium oxide (Y₂O₃), zirconium oxide (ZrO₂), strontiumtitanate oxide (SrTiO₃), zirconium silicon oxide (ZrSiO₄), hafniumzirconium oxide (HfZrO₄), strontium bismuth tantalate, (SrBi₂Ta₂O₉,SBT), lead zirconate titanate (PbZr_(x)Ti_(1-x)O₃, PZT), and bariumstrontium titanate (Ba,Sr_(1-x)TiO₃, BST), wherein x is between 0 and 1.

Thereafter, a conductive material layer 116 is formed on the high-klayer 114 in the first and second areas 10 and 20. The conductivematerial layer 116 includes polysilicon, amorphous silicon or acombination thereof, and the forming method thereof includes performinga suitable deposition process such as CVD.

Referring to FIG. 1F, the conductive material layer 116 and the high-klayer 114 are patterned, so as to form a conductive layer 116 a and anunderlying high-k layer 114 a on the substrate 100 in the first area 10,and simultaneously form a conductive layer 116 b and an underlyinghigh-k layer 114 b on the substrate 100 in the second area 20. Thepatterning step includes performing a photolithography and etchingprocess. In an embodiment, this patterning step can simultaneouslyremove a portion of the ONO dielectric layer 110, so the remaining ONOdielectric layer 110 a is formed below the high-k layer 114 a. In anembodiment, this patterning step can simultaneously remove a portion ofthe interfacial layer 112, so the remaining interfacial layer 112 a isformed below the high-k layer 114 b. In an embodiment, this patterningstep can simultaneously remove a portion of the pad oxide layer 101, sothe remaining pad oxide layer 101 a is formed between the ONO dielectriclayer 110 a and the substrate 100.

Thereafter, a dielectric layer 126 is formed around the conductivelayers 116 a and 116 b. In an embodiment, the dielectric layer 126surrounds the sidewalls and exposes the tops of the conductive layers116 a and 116 b. The dielectric layer 126 includes silicon oxide,borophosphosilicate glass (BPSG), phosphosilicate glass (PSG), undopedsilicate glass (USG), fluorosilicate glass (FSG), spin-on glass (SOG),or a low-k material with a dielectric constant lower than about 4. Themethod of forming the dielectric layer 126 includes perform a spincoating process or a suitable deposition process such as CVD. In anembodiment, before the step of forming the dielectric layer 126, spacers120 a and 120 b can be respectively formed on the sidewalls of theconductive layers 116 a and 116 b, and an etch sop layer 124 can beformed between the dielectric layer 126 and each of the spacers 120 aand 120 b and between the dielectric layer 126 and the substrate 100.

Referring to FIG. 1G, the conductive layers 116 a and 116 b are removedto form trenches 128 a and 128 b in the dielectric layer 126. The stepof removing the conductive layers 116 a and 116 b includes performing anetching process. In this embodiment, the trenches 128 a and 128 brespectively expose the high-k layers 114 a and 114 b.

Referring to FIG. 1H, conductive layers 130 a and 130 b are respectivelyfilled in the trenches 128 a and 128 b. Each of the conductive layers130 a and 130 b includes metal. In an embodiment, each of the conductivelayers 130 a and 130 b includes a work function metal layer and a lowresistivity metal layer (not shown). The work function metal layerincludes titanium nitride (TiN), titanium carbide (TiC), tantalumnitride (TaN), tantalum carbide (TaC), tungsten carbide (WC), aluminumtitanium nitride (TiAlN), titanium aluminide (TiAl), zirconium aluminide(ZrAl), tungsten aluminide (WAl), tantalum aluminide (TaAl), hafniumaluminide (HfAl) or a combination thereof The low resistivity metallayer includes Cu, Al or an alloy thereof.

The method of forming the conductive layers 130 a and 130 b includesperforming at least one suitable deposition process such as CVD, so asto form a metal material layer (not shown) on the substrate 100 fillingin the trenches 128 a and 128 b in the first and second areas 10 and 20.Thereafter, a CMP process is performed by using the dielectric layer 126as a polish stop layer, so as to remove the metal material layer outsideof the trenches 128 a and 128 b. The fabrication of the semiconductordevice of the invention is thus completed.

In this embodiment, in the memory device in the first area 10, theinsulating layer 106 serves as a tunnel insulating layer, the conductivelayer 108 a serves as a floating gate, the ONO dielectric layer 110 aand high-k layer 114 a together serve as an inter-gate dielectric layer,and the conductive layer 130 a serves as a control gate. In the MOStransistor device in the second area 20, the high-k layer 114 b servesas a gate dielectric layer, and the conductive layer 130 b serves as ametal gate.

In the conventional method, the metal gate of a MOS transistor device isusually at a level lower than that of the control gate of a memorydevice, so the control gate is subjected to damage during the polishingstep to the metal gate in an integrated process of forming a memory celland a metal gate MOS transistor. However, in the present invention,since the control gate (e.g., conductive layer 130 a) in the first area10 is fabricated simultaneously and formed at substantially the samelevel with the metal gate (e.g., conductive layer 130 b) in the secondarea 20, so the control gate of the invention is free of the damageduring the polishing step to the metal gate.

In an embodiment, the high-k layer 114/114 a in the first area 10 can beoptionally removed after the step of forming the ONO dielectric layer110 and before the step of forming conductive material layer 116, orafter the step of removing the conductive layers 116 a and 116 b andbefore the step of filling the conductive layers 130 a and 130 b. Amemory device of FIG. 2 is thereby formed, wherein the conductive layer130 a is in physical and direct contact with the ONO dielectric layer110 a.

Besides, in the said embodiments, the control gate (e.g., conductivelayer 130 a) has a dimension or width greater than that of the floatinggate (e.g., conductive layer 108 a), as shown in FIG. 1H and FIG. 2, butthe present invention is not limited thereto. In another embodiment, thecontrol gate (e.g., conductive layer 130 a) and the floating gate (e.g.,conductive layer 108 a) can have substantially the same dimension orwidth, as shown in FIG. 3 and FIG. 4.

FIG. 5A to FIG. 5E are schematic cross-sectional views of a method offorming a semiconductor device according to another embodiment of thepresent invention. The method of FIG. 5A to FIG. 5E is similar to thatof FIG. 1A to FIG. 1H, in which the same reference numbers refer to thesame or like elements, and the difference between them lies in that thestep of forming an ONO dielectric layer is not implemented in the methodof FIG. 5A to FIG. 5E.

Referring to FIG. 5A, a substrate 100 having first and second areas 10and 20 is provided. The substrate 100 has a pad oxide layer 101 formedthereon. Besides, the substrate 100 has at least one opening 104 formedin the first area 10. An insulating layer 106 is formed on the surfaceof the opening 104. A conductive layer 108 a is filled in the opening104. The surface of the conductive layer 108 a is no higher than thesurface of the substrate 100. In this embodiment, the surface of theconductive layer 108 a is substantially coplanar with the surface of thesubstrate 100. Thereafter, at least two doped regions 113 are formed inthe substrate 100 beside the conductive layer 108 a. In an embodiment,after the step of forming the doped regions 113, an etching process isperformed to remove the pad oxide layer 101 on the substrate 100 in thefirst and second areas 10 and 20.

Referring to FIG. 5B, an interfacial layer 112 is formed on thesubstrate 100 in the first and second areas 10 and 20. The interfaciallayer 112 includes silicon oxide, and the forming method thereofincludes preforming a thermal oxidation process or a suitable depositionprocess such as CVD.

Thereafter, a high-k layer 114 is formed on the substrate 100 in thefirst and second areas 10 and 20. In an embodiment, the high-k layer 114covers the interfacial layer 112 in the first and second areas 10 and20. The method of forming the high-k layer 114 includes performing asuitable deposition process such as CVD. In an embodiment, the high-klayer 114 can be a high-k layer with a dielectric constant greater thanabout 4, greater than about 7 or even greater than about 10. Forexample, the high-k layer 114 includes metal oxide.

Thereafter, a conductive material layer 116 is faulted on the high-klayer 114 in the first and second areas 10 and 20. The conductivematerial layer 116 includes polysilicon, amorphous silicon or acombination thereof, and the forming method thereof includes performinga suitable deposition process such as CVD.

Referring to FIG. 5C, the conductive material layer 116 and the high-klayer 114 are patterned, so as to form a conductive layer 116 a and anunderlying high-k layer 114 a in the first area 10, and simultaneouslyform a conductive layer 116 b and an underlying high-k layer 114 b onthe substrate in the second area 20. The patterning step includesperforming a photolithography and etching process. In an embodiment,this patterning step can simultaneously remove a portion of theinterfacial layer 112, so the remaining interfacial layer 112 a isformed below the high-k layer 114 b, and the remaining interfacial layer112 b is formed between the high-k layer 114 a and the conductive layer108 a. In an embodiment, the interfacial layer 112 b is further formedbetween the high-k layer 114 a and the substrate 100.

Thereafter, a dielectric layer 126 is formed around the conductivelayers 116 a and 116 b. In an embodiment, the dielectric layer 126surrounds the sidewalls and exposes the tops of the conductive layers116 and 116 b. In an embodiment, before the step of forming thedielectric layer 126, spacers 120 a and 120 b can be respectively formedon the sidewalls of the conductive layers 116 a and 116 b, and an etchstop layer 124 can be formed between the dielectric layer 126 and eachof the spacers 120 a and 120 b and between the dielectric layer 126 andthe substrate 100.

Referring to FIG. 5D, the conductive layers 116 a and 116 b are removedto form trenches 128 a and trench 128 b in the dielectric layer 126. Themethod of forming the conductive layers 116 a and 116 b includesperforming an etching process. In this embodiment, the trenches 128 aand 128 b respectively expose the high-k layers 114 a and 114 b.

Referring to FIG. 5E, conductive layers 130 a and 130 b are respectivelyfilled in the trenches 128 a and 128 b. Each of the conductive layers130 a and 130 b includes metal. In an embodiment, each of the conductivelayers 130 a and 130 b includes a work function metal layer and a lowresistivity metal layer. The fabrication of the semiconductor device ofthe invention is thus completed.

In this embodiment, in the memory device in the first area 10, theinsulating layer 106 serves as a tunnel insulating layer, the conductivelayer 108 a serves as a floating gate, the interfacial layer 112 b andhigh-k layer 114 a together serve as an inter-gate dielectric layer, andthe conductive layer 130 a serves as a control gate. In the MOStransistor device in the second area 20, the high-k layer 114 b servesas a gate dielectric layer, and the conductive layer 130 b serves as ametal gate.

In this embodiment, the control gate (e.g., conductive layer 130 a) inthe first area 10 and the metal gate (e.g., conductive layer 130 b) inthe second area 20 are formed simultaneously. Besides, the high-kdielectric layer 114 a between the control gate and the floating gate inthe first area 10 is formed simultaneously with the high-k layer 114 bbelow the metal gate in the second area 20.

In an embodiment, the control gate (e.g., conductive layer 130 a) has adimension or width greater than that of the floating gate (e.g.,conductive layer 108 a), as shown in FIG. 5E, but the present inventionis not limited thereto. In another embodiment, the control gate (e.g.,conductive layer 130 a) and the floating gate (e.g., conductive layer108 a) have substantially the same dimension or width, as shown in FIG.6.

The said embodiments in which the fabricating process of the memorydevice of the invention is integrated with that of the metal gate(high-k first) process are provided for illustration purposes, and arenot construed as limiting the present invention. It is appreciated bypeople having ordinary skill in the art that the fabricating process ofthe memory device of the invention can be integrated with that of themetal gate (high-k last) process.

The memory device structures of the present invention are illustratedbelow with reference to FIG. 1H, FIG. 2 to FIG. 4, FIG. 5E and FIG. 6.

The memory device of the invention includes a first gate (e.g.,conductive layer 108 a), a second gate (e.g., conductive layer 130 a), atunnel insulating layer (e.g., insulating layer 106) and an inter-gatedielectric layer. The first gate is buried in the substrate 100. Thesecond gate is disposed on the substrate 100, and the second gateincludes metal. The tunnel insulating layer is disposed between thefirst gate and the substrate 100. The inter-gate dielectric layer isdisposed between the first gate and the second gate. In an embodiment,the inter-gate dielectric layer is constituted by the ONO dielectriclayer 110 a and the high-k layer 114 a, as shown in FIG. 1H and FIG. 3.In another embodiment, the inter-gate dielectric layer is merelyconstituted by the ONO dielectric layer 110 a, as shown in FIG. 2 andFIG. 4. In yet another embodiment, the inter-gate dielectric layer isconstituted by the interfacial layer 112 b and the high-k layer 114 a,as shown in FIG. 5E and FIG. 6.

In an embodiment, the dimension of the second gate is greater than thatof the first gate, and the inter-gate dielectric layer is furtherdisposed between the second gate and the substrate 100, as shown in FIG.1H, FIG. 2 and FIG. 5E. In another embodiment, the first and secondgates have substantially equal dimension, as shown in FIG. 3, FIG. 4 andFIG. 6.

Besides, the memory device of the invention further includes two dopedregions 113 as source/drain regions disposed in the substrate 100 besidethe first gate. In an embodiment, the depth of the first gate is greaterthan the depth of the doped regions 113

In summary, in the present invention, a floating gate in a cell area isburied in a substrate, and a control gate is fabricated simultaneouslyand formed at the same levels with a metal gate in a periphery area. Bysuch manner, the control gate of the invention is not damaged during thepolishing step to the metal gate. Moreover, in the present invention, amemory device and a MOS transistor device can be simultaneouslyfabricated with the semiconductor process for forming a metal gate, sothe process cost is significantly reduced and the competiveness isgreatly improved.

The present invention has been disclosed above in the preferredembodiments, but is not limited to those. It is known to persons skilledin the art that some modifications and innovations may be made withoutdeparting from the spirit and scope of the present invention. Therefore,the scope of the present invention should be defined by the followingclaims.

1. A memory device, comprising: a first gate, buried in an opening of asubstrate; a second gate, disposed on the substrate, wherein the secondgate comprises metal; and an inter-gate dielectric layer, disposedbetween the first gate and the second gate and covering a top corner ofthe opening.
 2. The memory device of claim 1, wherein a dimension of thesecond gate is greater than a dimension of the first gate, and theinter-gate dielectric layer is further disposed between the second gateand the substrate.
 3. The memory device of claim 1, wherein theinter-gate dielectric layer comprises an oxide-nitride-oxide (ONO)dielectric layer, a high-dielectric-constant (high-k) layer having adielectric constant of greater than about 10 or a combination thereof.4. The memory device of claim 3, wherein the high-k layer comprisesmetal oxide.
 5. The memory device of claim 1, further comprising atunnel insulating layer disposed between the first gate and thesubstrate.
 6. The memory device of claim 1, further comprising at leasttwo doped regions disposed in the substrate beside the first gate. 7.The memory device of claim 6, wherein a depth of the first gate isgreater than a depth of the doped regions.
 8. A memory device,comprising: a first gate, buried in the substrate; a second gate,disposed on the substrate, wherein the second gate comprises metal; andan inter-gate dielectric layer, disposed between the first gate and thesecond gate, wherein the inter-gate dielectric layer comprises a high-klayer having a dielectric constant of greater than about
 10. 9. Thememory device of claim 8, wherein a dimension of the second gate isgreater than a dimension of the first gate, and the inter-gatedielectric layer is further disposed between the second gate and thesubstrate.
 10. The memory device of claim 8, further comprising aninterfacial layer disposed between the high-k layer and the first gate.11. The memory device of claim 8, further comprising a tunnel insulatinglayer disposed between the first gate and the substrate.
 12. The memorydevice of claim 8, further comprising at least two doped regionsdisposed in the substrate beside the first gate.
 13. The memory deviceof claim 12, wherein a depth of the first gate is greater than a depthof the doped regions.
 14. A method of forming a memory device,comprising: providing a substrate having a first area and a second area,wherein the substrate has at least one opening formed in the first area;forming an insulating layer on a surface of the opening; filling a firstconductive layer in the opening; and forming a first dielectric layerand a second conductive layer on the first conductive layer, wherein thefirst dielectric layer is disposed between the first conductive layerand the second conductive layer and covers a top corner of the opening.15. The method of claim 14, further comprising: forming a seconddielectric layer around the second conductive layer; removing the secondconductive layer to form a trench in the second dielectric layer; andfilling a third conductive layer in the trench.
 16. The method of claim15, wherein the second conductive layer comprises polysilicon, amorphoussilicon or a combination thereof, and the third conductive layercomprises metal.
 17. The method of claim 15, wherein the thirdconductive layer in the first area and a metal gate in the second areaare formed simultaneously.
 18. The method of claim 14, wherein the firstdielectric layer comprises an ONO dielectric layer, a high-k layerhaving a dielectric constant greater than about 10 or a combinationthereof.
 19. The method of claim 14, wherein a surface of the firstconductive layer is lower than a surface of the substrate.
 20. Themethod of claim 14, wherein the first dielectric layer in the first areaand a high-k layer below a metal gate in the second area are formedsimultaneously.